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Thermodynamic characterization of separations on alkaline-stable silica-based C18 columns: Why basic solutes may have better capacity and peak performance at higher pH
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Physical and Analytical Chemistry, Surface Biotechnology.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Physical and Analytical Chemistry, Surface Biotechnology.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Physical and Analytical Chemistry, Surface Biotechnology.
2007 (English)In: Journal of Chromatography A, ISSN 0021-9673, E-ISSN 1873-3778, Vol. 1163, no 1-2, 177-189 p.Article in journal (Refereed) Published
Abstract [en]

A systematic study was made to explain the large improvements in separation performance and capacity of basic compounds at alkaline conditions. The adsorption of three probe components was investigated on four alkaline-stable silica-based C18 columns at three different pH-levels: 3, 7 and 11. The probes were 3-phenyl-1-propanol (neutral), 2-phenylbutyric acid (acidic) and metoprolol (basic). Adsorption isotherms were acquired over a broad concentration range, in order to detect both high and low energy sites. Before the choice of the proper adsorption isotherm model, the adsorption energy distribution (AED) was calculated yielding the number of different kinds of interaction sites between the solute and the stationary phase. The neutral probe was entirely unaffected by pH and its AED was unimodal (one site) indicating homogenous adsorption. For the acidic probe the interactions were unimodal at pH 3 where the probe is uncharged and at least bimodal (two sites) at pH 7 and 11 where the probe is charged. For the basic probe, the interactions were heterogeneous at both pH 3 and 11. The equilibrium constants of the high and low energy sites were different by a factor of 55–100 at pH 3 and only 6–7 at pH 11. The difference in saturation capacities between the two sites was much smaller at pH 11 where 20% of the total capacity is from the high energy site, as compared to pH 3 where the high energy site was only 2–5% of the total capacity. This explains why peaks of amines (basic solutes) tail at low pH while their peaks are symmetrical at alkaline pH. The Langmuir model fit the unimodal data and the bi-Langmuir model fit the bimodal AED data. The calculated band profiles based on these parameters agreed excellently with the experimental data. The electrostatic-modified Langmuir, on the other hand, did not describe this adsorption process well.

Place, publisher, year, edition, pages
2007. Vol. 1163, no 1-2, 177-189 p.
Keyword [en]
Adsorption energy distribution, Adsorption isotherm, Alkaline conditions, Column characterization, Frontal analysis of characteristic points, Chromatography, FACP
National Category
Chemical Sciences
URN: urn:nbn:se:uu:diva-96958DOI: 10.1016/j.chroma.2007.06.026ISI: 000249310800018PubMedID: 17612549OAI: oai:DiVA.org:uu-96958DiVA: diva2:171706
Available from: 2008-04-04 Created: 2008-04-04 Last updated: 2011-04-13Bibliographically approved
In thesis
1. Development of Methods for Phase System Characterization in Liquid Chromatography
Open this publication in new window or tab >>Development of Methods for Phase System Characterization in Liquid Chromatography
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The aim of this thesis is first and foremost to improve the fundamental knowledge of nonlinear and preparative separation theory by focusing on some of the remaining “white spots” on the theoretical chromatographic map. Secondly, the acquired knowledge is used to develop, validate and execute new methods for phase characterization in liquid chromatography. The methodology used in this thesis is a combination of experiments, fundamental nonlinear theory and systematic computer simulations.

A fundamental knowledge of the molecular interactions between the compounds to be separated and the separation media requires the determination of adsorption isotherms over a broad concentration range to give a complete picture of all interactions in the separation system - weak as well as strong. In addition, such adsorption data is essential for optimization in preparative chromatography.

For the first time, it has been experimentally shown that the injected molecules are not present in the detected peak when a small excess of molecules are injected into a chromatographic system equilibrated with a constant stream of identical molecules. Several experimental procedures for this method were developed such as (i) the optimal injection strategy and (ii) different labeling methods for visualizing the injected molecules. Remarkable phenomena in the single-component case, such as invisible peak deformation and deformed (invisible) frontal chromatograms, are reported, investigated, and explained. This phenomenon has asides from its future practical implementation, also a large didactic value.

The accuracy of the ECP method is experimentally improved, and used to characterize the separation of protolytic compounds at different pH on modern commercially available silica and hybrid silica column packing materials. That investigation enables us to answer why basic compounds give a much more compact preparative peak profile at pH 11 than they yields at lower pH.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2008. 69 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 416
Analytical chemistry, Liquid chromatography, Adsorption isotherms, Adsorption energy distribution, Tracer pulse method, Elution by characteristic points, Peak deformation, Validation, Hybrid column material, Alkaline separation, Injection-profiles, Analytisk kemi
urn:nbn:se:uu:diva-8597 (URN)978-91-554-7145-3 (ISBN)
Public defence
2008-04-25, B41, BMC, Husargatan 3, Uppsala, 13:15 (English)
Available from: 2008-04-04 Created: 2008-04-04 Last updated: 2009-03-20Bibliographically approved

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